Made template for service VM unique using generic driver

Without this commit, using multibackend installation with more than
one generic driver, for all of them will be used only one service VM,
in case of default value for template of service VM.
It can cause ssh connection error to service VM by backend, that
didn't create it and has different username or password.

Closes-Bug: #1314296
Change-Id: I9d307ca61e8306a301de403738296823d25a2a62